The present invention is generally related to optic fiber networks, and in particular, provides an attenuator for controllably varying the strength of an optical signal.
In fiber optic networks, light signals are transmitted along optical fibers to transfer information from one location to another. It is often desirable to tailor the power of optical signals within optical fiber networks. For example, the individual components of an optical fiber network may be tested using a low power optical signal to simulate fiber optic telecommunications or data communications over a long distance. Tailoring of optical signal strengths is desirable within automatic optical testing systems, optical signal routing systems, optical sensor arrays, and the like.
Although the propagation of light signals within optical fibers is somewhat analogous to the transmission of electronic data along metal wires, manipulating the strength of an optical signal within an optical fiber is more problematic than varying the strength of an electrical signal along a wire. Electrical signal strength can be varied simply by generating a different input voltage, or by coupling the wire to a ground through a variable resistor. Optical signals are typically generated by laser diodes, and, unfortunately, laser diodes can be less flexible than electrical signal generators.
Furthermore, once the signal has entered the optical fiber, selectably and repeatably removing or blocking a portion of the optical signal is more challenging than simply varying a resistor. Although optical signals can be filtered while they are transmitted between aligned ends of optical fibers, such arrangements are susceptible to both misalignment and contamination. Single mode optical fibers are particularly susceptible to such misalignment and contamination, as their core diameters are typically as small as 2 to 10 xcexcm. Hence, even minute particles can block a substantial portion of the signal.
In light of the above, it would be desirable to provide improved structures and methods for attenuating optical signals. It would be particularly desirable if such improved structures avoided precise alignment requirements, and were controllably and repeatably variable throughout a wide range of attenuation values.
The present invention provides improved optical attenuators which generally filter an optical signal after the signal has been radially expanded by a collimating lens. Such signal expansion decreases the attenuator""s sensitivity to misalignment and contamination. In a preferred arrangement, the optical signals are expanded and refocused by a single GRIN lens, with the signal passing twice through a variable filter: a first time when transmitted from the GRIN lens, and a second time when travelling back toward the GRIN lens from a reflector. Assembly of the attenuator is simplified by aligning both the input and output fibers within a single sleeve. An electrical signal which indicates the amount of attenuation can be provided by using a potentiometer actuation mechanism to vary the filter between the GRIN lens and the reflector, resulting in a low cost, highly accurate variable attenuator.
In a first aspect, the present invention provides a variable attenuator comprising an electrical component having a body and an element. The body defines a path along which the element moves to vary an electrical characteristic of the component. An output optical fiber is disposed in a optical path of an optical signal from an input optical fiber, and an optical filter having a filter density which varies between a lower density region and a higher density region is coupled to the component. An effective region of the filter is disposed in the optical path between the input fiber and the output fiber. The element of the component moves along the path as the effective filter region moves between the lower density region and the higher density region of the filter, thereby allowing the electrical characteristic of the wiper to indicate effective attenuation.
Preferably, the variable attenuator of the present invention makes use of a potentiometer actuation mechanism, ideally by mounting the filter to a wiper of the potentiometer. More generally, any electrical component having a movable element might be used (for example, a variable capacitor, a potentiometer, etc.) by mounting the filter to either the body or the movable element, and by mounting the optical fibers to the other. The element may rotate and/or move linearly, while a linear screw element actuation mechanism is particularly preferred.
In another aspect, the present invention provides an attenuator comprising a sleeve having a first end and a second end. An input optical fiber is disposed within the sleeve, and has an end adjacent to the second end of the sleeve. An output optical fiber is also disposed within the sleeve with an end adjacent to the second end of the sleeve. A GRIN lens is adjacent to the second end of the sleeve. The GRIN lens has a length of approximately a quarter pitch, and is aligned to expand an optical signal from the input fiber. A reflector is disposed in the optical path of the optical signal from the GRIN lens, and is aligned to direct the signal back through the GRIN lens and into the output fiber. An optical filter is disposed in the expanded optical path between the GRIN lens and the reflector. Typically, the filter will have a variable effective filter density to vary the strength of the optical signal which enters the output fiber.